An unusual form of hepatitis virus, hepatitis D (HDV), also called .delta. agent, was discovered in 1977 by Rizzetto, M., et al, Gut (1977) 18:997-1003. The virus was detected as a new antigen/antibody system by immunofluorescence in liver cells of patients infected with hepatitis B. Indeed, subsequent investigation showed that hepatitis D virus is dependent upon concomitant infection with hepatitis B in order to replicate. The nature of the helper function is not as yet understood. However, the HDV apparently contains a single-stranded RNA genome surrounded by a ".delta. antigen" protein, which is in turn surrounded by hepatitis B surface antigen (HBsAg) in a 35-37 nm particulate configuration (Rizzetto, M. et al, Proc Natl Acad Sci USA (1980) 77:6124-6128: Bonino, F., et al, Hepatology (1981) 1:127-131). Thus, the DNA produced during infection will have a "genomic" strand and a complementary strand.
The epidemiology and mode of transmission appears to be similar for HDV to that of hepatitis B (HBV), in that it is transmitted through blood transfusion and by close direct contact of body fluids. Three patterns of HDV (or .delta.) infection have been identified: acute .delta. infection superimposed on chronic B, chronic .delta. superimposed on chronic B, and simultaneous acute .delta. and hepatitis B infections (Schiff, E. R., et al, Diagnostic Medicine (March 1985) 17-22). While the disease was originally identified in the Mediterranean basin, it appears to be spreading worldwide (Jacobson, I. M., et al, Hepatology (1985) 5:188-191). A review of the demographic and epidemiological aspects of this disease is also found in Rizzetto, M. et al, J Hepatol (1985) 1:187-193.
Although the course of the disease has been well characterized and the general structure of the virion is understood, no information has previously been available as to the genetic structure of the virus, nor has the nature of the .delta. antigen been characterized. The only available assay to detect the presence of the disease by using blood samples is an immunoassay marketed in Europe, which has not yet received FDA approval in the United States. Previous detection methods were limited to direct immunofluorescence in the nuclei of hepatocytes in biopsy specimens. One form of the assay is based on the ability of antibody in test serum to block binding of labeled IgG anti-.delta. to .delta. antigen per se. Another configuration relies on the ability of IgM anti-.delta. from the test serum to bind antihuman IgM (specific for .mu. chain) fixed to the solid phase, followed by the addition of standard .delta. antigen and labeled IgM anti-.delta. so that the presence of IgM anti-.delta. in the test serum (along with the added .delta. antigen) permits binding of labeled anti-.delta. IgM. Neither of these tests requires analysis of, or knowledge of, the .delta. antigen protein structure or HDV genomic structure.
It is now possible to design efficient probes for diagnosis of the disease by DNA hybridization, as well as to generate recombinant proteins usable as vaccines and as reagents in diagnostic testing. In addition, the recombinantly produced proteins can be used to generate antibodies useful for diagnosis or for passive therapy.